1. The Field of the Invention
The present invention is related to surface cleaning and preparation of metal substrates in order to provide improved bonding to those substrates. The present invention is further related to a new class of adhesion promoters, including specifically alkoxy silanes, and methods of synthesis of those materials.
2. Technical Background
In constructing various structures from metals it is important to have the capability of bonding to metal surfaces. This includes bonding metal surfaces to other metal surfaces, as well as bonding non-metal materials to metal surfaces. In many applications it is possible to use simple mechanical bonding mechanisms, such as bolts, screws, or rivets. In other applications, concerns over the added weight of mechanical fasteners make the use of adhesive more viable. Various adhesives are known and commonly used in the art of bonding metals together or bonding non-metal materials to metals. For example, various epoxy-based adhesives are widely used for these applications.
When metals are bonded using an adhesive it is generally important to provide the strongest possible bond. In the past it was difficult to assure a strong bond when using adhesive. For example, processing conditions during bond fabrication often cause dramatic reductions in bond strength.
Metal and metal alloys such as titanium, titanium alloys, steel, and steel alloys are generally considered difficult to bond to. Some metals, for example, have a propensity to form a weak hydrated surface layer of metal oxide. With these metals the surface morphology and hence, adhesive bond durability, is dependent upon the type of surface treatment received prior to bonding.
A widely used treatment for metal surfaces involves vapor degreasing and grit blasting. However, these methods do not provide sufficient initial bond strength or bond durability for adhesives such as phenylethynyl terminated polyimides as evidenced by significant metal interfacial failure. Therefore, the bondline properties are very sensitive to the relative humidity and process times from surface preparation to bonding. Moisture can also penetrate the bondline during aging, resulting in degradation of bondline properties and interfacial failures.
Available cleaning solvents used in this process have become more restrictive because of environmental regulation on chemical waste disposal. Thus, the combination of process sensitivity, marginal bond durability, and environmental constraints, raises concerns over continued use of traditional surface treatment processes.
There are limited alternative methods of metal surface treatment and preparation. Some of these other methods of metal surface preparation involve formation of stable, moisture-resistant oxide layers. These methods include sulfuric, chromic, and phosphoric acid anodization. These electrolytic processes inhibit the further growth of corrosion surface layers and enhance initial bond strength and bond durability. In addition, the phosphoric acid process produces a honeycomb surface which is believed to enhance bond strength through mechanical interlocking.
These processes, however, generally consist of a complex series of treatments including degreasing, alkaline cleaning, acid etching, acid anodization, and in some instances, a post treatment process including primers and coatings. These processes use heavy metals, acids, caustics and other hazardous and toxic chemicals which pose handling and disposal problems. Clearly, environmental constraints limit the usefulness of these traditional surface treatment processes.
Accordingly, what is needed in the art are effective and efficient methods of surface preparation and treatment of titanium, steel, and other metals, in order to provide stable adhesive bonding to metal substrates. In that regard it would be a significant advancement in the art to provide methods of surface treatment and preparation which were relatively simple, and which used readily available materials. It would be a related advancement in the art to provide such methods which employed materials that did not present a significant environmental hazard. It would be an advancement in the art to provide improved materials which promote adhesion. It has been discovered, as discussed below, that materials such as 4-phenylethynyl-N [3-triethoxysilylpropyl] phthalimide can be employed to promote adhesion. It would also be a significant advancement in the art to provide improvement methods of synthesizing this material for use as an adhesion promoter.
Such methods are disclosed and claimed herein.